1. Field of the Invention
The subject invention relates to sonar systems and, more particularly, to a technique for quantization of delays in digital beamformers used in such systems.
2. Description of Related Art
In the prior art, beamformers comprising an arbitrary array of hydrophones are known. Such arrays permit directional sensitivity. In order to sense a signal wavefront at a selected arrival angle to the hydrophone line, the output signal of each hydrophone is appropriately delayed so that the collective outputs add coherently, in phase. The resultant output is characterized by a main lobe about a maximum response axis in the desired look direction and several side lobes in other directions.
In a digital beamformer, the outputs of the respective hydrophones are sampled by analog to digital converters and fed to a digital processor for storage and summing. A shading coefficient may also be applied to each selected hydrophone output prior to summing, in order to reduce the sidelobes of the system.
In digital beamforming systems, the exact values of delay used to steer a beam in a given direction must be quantized to multiples of the system sample interval. Quantization in effect determines which hydrophone output sample will be associated with a given steering angle.
Most existing digital beamformers quantize the time delays by simply rounding the full precision values. While this approach assures that the delays are as close as possible to the true values within the available accuracy of the beamformer, it does not relate the quantization to the resulting steering angle In fact, for certain array geometries, rounding can be shown to be a poor choice from the point of view of preserving steering accuracy. The same is true of delay quantization by truncating the exact values, as is done in some systems.
Existing quantization techniques, such as truncation or rounding thus contribute to deviations from the desired steering angle In systems where high accuracy is required, e.g., the target is at a relatively long distance, it is desirable to eliminate as many errors as possible to gain steering accuracy. In other systems, a more accurate quantization technique could produce acceptably accurate steering from a courser quantization, thereby reducing the sample rate and hardware requirements, and hence the expense of the system.